How Much Protein is Enough?

Protein deficiencies usually occur only when a horse is on very poor pasture or hay with no other supplemental feed, for a prolonged period of time.

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Are you confused about how much protein your horse should consume? Don't worry: You're not alone. Here's what you need to know about protein excesses and deficiencies in horse diets, and how to determine how much protein your horse needs each day.

Excesses and Deficiencies

Horses that receive inadequate amounts of protein in their diets can suffer a number of ill effects, including decreased growth and development in youngsters, and reduced appetite, body tissue weight loss, slow hoof growth, energy deficit, and a poor hair coat with reduced shedding in adults. Pregnant mares with protein deficiencies may become more prone to abortions, and lactating mares suffer declines in milk production. Muscle deterioration, especially in the large muscle groups of the hindquarters, also might be evident, and some horses will begin eating manure. The reduced food intake of a depressed, protein-deficient horse can become a vicious cycle, making efforts to correct the condition difficult.

Protein deficiencies usually occur only when a horse is on very poor pasture or hay with no other supplemental feed, for a prolonged period of time. With a corrected diet, most of the signs of protein deficiency in adult horses can be turned around in as little as a week. The damage done to a young, growing horse, however, can be more serious.

More common, and equally damaging, is an excess of protein in the diet, especially in mature horses that have been fed by owners laboring under the misunderstanding that protein equals energy. Here’s what happens: Protein not used immediately by the horse’s system is broken down to release the nitrogen atoms (the rest of the molecule being stored), and those nitrogen atoms become bound up as ammonia and urea molecules. The ammonia and urea eventually are excreted in the urine, which leads to increased water intake, increased urination, and a noticeably strong ammonia smell in the stall. And before ammonia and urea can be excreted in the urine, they must be filtered out of the blood—a process that, over time, can tax the kidneys.

Decreased athletic performance is another possible outcome of a high-protein diet. There’s some evidence that excess protein can interfere with calcium absorption in some species, and the absorption of both calcium and phosphorus in weanlings. Researchers differ, however, on how much damage a high-protein diet can cause, and how long a horse must be fed such a diet before the effects (if any) are noticeable. There is stronger evidence for the detrimental effect of excess protein in growing horses—in one study, weanlings and yearlings fed a diet 25% higher in protein than normal suffered slower rates of growth overall and higher incidences of developmental bone and joint problems.

How Much Is Enough?

So what is an appropriate level of protein for your horse? Continuing research is changing that answer all the time, but there are some general guidelines. The amount of crude protein needed in the diet depends on the needs of the individual horse (the most pivotal question being, is he still growing?), the digestibility or “bioavailability” of the protein, and the amount of the diet consumed. As a rule, though, a value of 0.60 g of digestible protein (1.26 g of crude protein) per kilogram of body weight per day is appropriate for most adult horses for maintenance metabolism. Broodmares in their trimester of pregnancy don’t really need supplemental levels of protein, but from the fifth month on, when the fetus does 60% to 65% of its growing their protein requirements increase.

Lactation (nursing) also demands higher protein levels; the protein content of mare’s milk is highest right after foaling and decreases gradually as the lactation period progresses. In one study, nursing broodmares fed less than 2.8 g of crude protein per kilogram of bodyweight per day lost weight and produced less milk than mares fed at least 3.2 g of crude protein per kilo of bodyweight per day.

Some researchers feel that during the breeding season, stallions also can benefit from a higher level of dietary protein. And hard exercise (such as racing, three-day eventing, or endurance racing) does increase the need for protein in the diet of adult horses, to support increased muscle development and mass, and to replace nitrogen lost in sweat.

Which feeds provide the best protein? Animal sources, such as milk and egg protein, and even fish and meat meal, offer the best amino acid profile and the highest levels of lysine. Milk protein is often used as the primary protein source for foal feeds, but because it is quite expensive (and because adult horses are far less sensitive to protein quality differences), it’s rarely found in feeds for mature animals.

Among the plant sources, soybean and canola meal are the next best things—they are the only two plant protein products that contain adequate amounts of lysine and methionine. Other common protein sources, such as linseed meal and cottonseed meal, have poor amino acid profiles and are generally supplemented with amino acids added by the feed manufacturer.

Grains themselves (such as oats, corn, and barley) can contain between 8% and 20% protein, but it’s of poor quality—which is the reason most feed companies add a higher-quality protein supplement to their “balanced” feeds (sweet feeds, pellets, and other pre-mixed rations). If the manufacturers have done their job, the feed should contain at least 0.65% lysine (on a dry matter basis). If this level isn’t present, more feed will be required to get the same results (particularly with young, growing horses). Protein, while a crucial part of your horse’s diet, has to be viewed in the proper perspective—as just one part of a working whole in the nutrition scheme.

About the Author

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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